The invention is applicable in the heat treatment of any suitable traveling web, and is especially useful in the annealing of long, continuous strips or sheets of aluminum or steel.
Those skilled in the art realize that the most economical method of annealing involves the processing of identical gauge materials which require about the same annealing temperatures. Unfortunately, this is not always possible. The next most economical method involves the continuous annealing of different gauge materials which require about the same annealing temperatures, since it is simpler and more energy efficient to adjust the line speeds at which the material is annealed, rather than radically alter or fluctuate the temperature within the annealing furnace. It is, therefore, not uncommon to attach to the trailing end of a strip of metal, the leading end of a strip of metal having a heavier or lighter gauge.
It is also well known that the processing temperatures in an annealing furnace are maintained higher than the temperature to which a strip of metal is desired to be heated to generally speed up the annealing process. For example, the annealing furnace may be maintained at a temperature of about 1600.degree. F., when it is desired to heat a strip of metal to a temperature of about 1450.degree. F. In the aforementioned example, it can be appreciated that, if the time/temperature processing conditions are maintained between two joined metal strips of dissimilar gauges, then the beginning portion of the second or trailing metal strip will be improperly treated until the first or leading metal strip exits the furnace and the processing conditions can be adjusted to accommodate the different gauge of the second metal strip. Under such conditions, a thousand feet of the second metal strip may be wasted, because of the high line speeds at which the metal strip is moved through the annealing furnace.
This problem is oftentimes solved by using a dummy coil or strip of metal that is attached to the trailing end of the first strip of metal. The dummy strip is considered wasted material and is reused many times in such operations to gain time to allow the exodus of the first metal strip from the annealing furnace and subsequent adjustment of the line speed to accommodate the gauge of the second metal strip which is fastened to the trailing end of the dummy strip. This particular method works, but is uneconomical, since considerable time is lost in the annealing of the dummy strip. The invention is designed to overcome this problem by maximizing the annealing of metal strip while minimizing the waste of metal strip caused by improper heat treatment.
Briefly stated, the invention is in a method of controlling the temperatures of two continuous traveling webs which are joined together, require essentially the same heat treatment temperatures, and are of gauges sufficiently different to affect the time of treatment. The leading and trailing ends of these two webs are joined together by any suitable means. The processing temperature is reduced from a normally higher processing level to a desired lower level when the juncture of the two webs enters the chamber in which the heat treatment process of the webs is carried out, the lower temperature level being such that both webs will be properly heated. The line speed at which the webs travel through the heat treatment chamber is slowed down in accordance with the reduction in the processing temperature.
Once the leading web is free of the chamber, then the processing conditions can be readjusted to accommodate the new gauge of the trailing web. Thus, it can be appreciated that no web should be wasted because of overheating, as the annealing process is maintained continuous.